Alpha Aquarii

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α Aquarii
Aquarius constellation map.svg
Red circle.svg
Location of α Aquarii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Aquarius
Right ascension 22h 05m 47.03593s[1]
Declination −00° 19′ 11.4568″[1]
Apparent magnitude (V) 2.942[2]
Spectral type G2 Ib[3]
U−B color index +0.699[2]
B−V color index +0.971[2]
R−I color index +0.49[2]
Radial velocity (Rv)7.5[4] km/s
Proper motion (μ) RA: +18.25[1] mas/yr
Dec.: −9.39[1] mas/yr
Parallax (π)6.23 ± 0.19[1] mas
Distance520 ± 20 ly
(161 ± 5 pc)
Absolute magnitude (MV)–3.882[5]
Mass6.5 ± 0.3[3] M
Radius77 ± 15[6] R
Diameter77 ± 15[6] D
Luminosity3,000[7] L
Surface gravity (log g)1.76 ± 0.04[3] cgs
Temperature5,210 ± 100[3] K
Metallicity [Fe/H]+0.17[5] dex
Rotational velocity (v sin i)6.7 ± 1.5[7] km/s
Age53[3] Myr
Other designations
El Melik, Rucbah, Saad el Melik, Sadalmelek, Sadalmelik, Sadlamulk, α Aqr, Alpha Aquarii, Alpha Aqr, 34 Aquarii, 34 Aqr, BD−01 4246, FK5 827, HD 209750, HIP 109074, HR 8414, SAO 145862, WDS 22058-0019.[8][9][10][11]
Database references

Alpha Aquarii (α Aquarii, abbreviated Alpha Aqr, α Aqr), also named Sadalmelik,[12] is a single star in the constellation of Aquarius. The apparent visual magnitude of 2.94[2] makes this the second-brightest star in Aquarius. Based upon parallax measurements made during the Hipparcos mission, it is located at a distance of roughly 520 light-years (160 parsecs) from the Sun.[1]

It forms the primary or 'A' component of a double star designated WDS J22058-0019 (the secondary or 'B' component is UCAC2 31789179[13]).


α Aquarii (Latinised to Alpha Aquarii) is the star's Bayer designation. WDS J22058-0019 A is its designation in the Washington Double Star Catalog.

It bore the traditional name Sadalmelik, which derived from an Arabic expression سعد الملك (sa‘d al-malik), meaning "Luck of the king". The name Rucbah had also been applied to this star; though it shared that name with Delta Cassiopeiae.[11] It is only one of two stars with ancient proper names to lie within a degree of the celestial equator. The origin of the Arabic name is lost to history.[14] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[15] to catalogue and standardize proper names for stars. The WGSN approved the name Sadalmelik for Alpha Aquarii (WDS J22058-0019 A) on 21 August 2016, and it is now so included in the List of IAU-approved Star Names (Delta Cassiopeiae was given the name Ruchbah).[12]

In Chinese, 危宿 (Wēi Xiù), meaning Rooftop (asterism), refers to an asterism consisting of Alpha Aquarii, Theta Pegasi and Epsilon Pegasi.[16] Consequently, Alpha Aquarii itself is known as 危宿一 (Wēi Xiù yī, English: the First Star of Rooftop).[17]


With an age of 53 million years,[3] Alpha Aquarii has evolved into a supergiant with a stellar classification of G2 Ib.[3] It has 6.5[3] times as much mass as the Sun and has expanded to around 77[6] times the Sun's radius. It is radiating 3,000[7] times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 5,210 K.[3] At this heat, the star glows with the yellow hue of a G-type star.[18] Examination of this star with the Chandra X-ray Observatory shows it to be significantly X-ray deficient compared to G-type main sequence stars. This deficit is a common feature of early G-type giant stars.[7]

The visual companion (UCAC2 31789179) has an apparent visual magnitude of approximately 12.2. It is at an angular separation of 110.4 arcseconds from Alpha Aquarii along a position angle of 40°.[10]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357
  2. 2.0 2.1 2.2 2.3 2.4 Cousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", South African Astronomical Observatory Circulars 8: 59, Bibcode 1984SAAOC...8...59C
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Lyubimkov, Leonid S. et al. (February 2010), "Accurate fundamental parameters for A-, F- and G-type Supergiants in the solar neighbourhood", Monthly Notices of the Royal Astronomical Society 402 (2): 1369–1379, arXiv:0911.1335, Bibcode 2010MNRAS.402.1369L, doi:10.1111/j.1365-2966.2009.15979.x
  4. Wielen, R. et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb (Astronomisches Rechen-Institut Heidelberg) 35 (35): 1, Bibcode 1999VeARI..35....1W
  5. 5.0 5.1 Soubiran, C. et al. (2008). "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants". Astronomy and Astrophysics 480 (1): 91–101. arXiv:0712.1370. Bibcode 2008A&A...480...91S. doi:10.1051/0004-6361:20078788.
  6. 6.0 6.1 Nordgren, Tyler E. et al. (December 1999), "Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer", The Astronomical Journal 118 (6): 3032–3038, Bibcode 1999AJ....118.3032N, doi:10.1086/301114
  7. 7.0 7.1 7.2 7.3 Ayres, Thomas R.; Brown, Alexander; Harper, Graham M. (July 2005), "Chandra Observations of Coronal Emission from the Early G Supergiants α and β Aquarii", The Astrophysical Journal 627 (1): L53–L56, Bibcode 2005ApJ...627L..53A, doi:10.1086/431977
  8. HR 8414, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line October 3, 2008.
  9. NAME SADALMELIK -- Star in double system, database entry, SIMBAD. Accessed on line October 3, 2008.
  10. 10.0 10.1 Entry 22058-0019, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line November 18, 2008.
  11. 11.0 11.1 pp. 51, 148, Star-names and Their Meanings, Richard Hinckley Allen, New York: G. E. Stechert, 1899.
  12. 12.0 12.1 Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  13. "UCAC2 31789179 -- Star", SIMBAD (Centre de Données astronomiques de Strasbourg),, retrieved 2018-02-25
  14. Sadalmelik, Stars, Jim Kaler. Accessed on line October 3, 2008.
  15. IAU Working Group on Star Names (WGSN), International Astronomical Union,, retrieved 22 May 2016.
  16. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  17. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2010-08-18 at the Wayback Machine., Hong Kong Space Museum. Accessed on line November 23, 2010.
  18. "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, archived from the original on March 10, 2012,, retrieved 2012-01-16

External links